Experimental and Theoretical Study of the OH-Initiated Degradation of Piperidine under Simulated Atmospheric Conditions

The OH-initiated photo-oxidation of piperidine and the photolysis of 1-nitrosopiperidine were investigated in a large atmospheric simulation chamber and in theoretical calculations based on CCSD(T*)-F12a/aug-cc-pVTZ//M062X/aug-cc-pVTZ quantum chemistry results and master equation modeling of the pivotal reaction steps. The rate coefficient for the reaction of piperidine with OH radicals was determined by the relative rate method to be k_{OH-piperidine} = (1.19 ± 0.27) × 10^-10 cm³ molecule^-1 s^-1 at 304 ± 2 K and 1014 ± 2 hPa. Product studies show the piperidine + OH reaction to proceed via H-abstraction from both CH₂ and NH groups, resulting in the formation of the corresponding imine (2,3,4,5-tetrahydropyridine) as the major product and in the nitramine (1-nitropiperidine) and nitrosamine (1-nitrosopiperidine) as minor products. Analysis of 1-nitrosopiperidine photolysis experiments under natural sunlight conditions gave the relative rates j_rel = j_{1-nitrosoperidine}/j_NO = 0.342 ± 0.007, k₃/k_4a = 0.53 ± 0.05 and k₂/k_4a = (7.66 ± 0.18) × 10^-8 that were subsequently employed in modeling the piperidine photo-oxidation experiments, from which the initial branchings between H-abstraction from the NH and CH₂ groups, k_N-H/k_tot = 0.38 ± 0.08 and k_C/k_tot = 0.49 ± 0.19, were derived. All photo-oxidation experiments were accompanied by particle formation that was initiated by the acid-base reaction of piperidine with nitric acid. Primary photo-oxidation products including both 1-nitrosopiperidine and 1-nitropiperidine were detected in the particles formed. Quantum chemistry calculations on the OH initiated atmospheric photo-oxidation of piperidine suggest the branching in the initial H-abstraction routes to be ∼35% N¹, ∼50% C², ∼13% C³, and ∼2% C⁴. The theoretical study produced an atmospheric photo-oxidation mechanism, according to which H-abstraction from the C² position predominantly leads to 2,3,4,5-tetrahydropyridine and H-abstraction from the C³ position results in ring opening followed by a complex autoxidation, of which the first few steps are mapped in detail. H-abstraction from the C⁴ position is shown to result mainly in the formation of piperidin-4-one and 2,3,4,5-tetrahydropyridin-4-ol, whereas H-abstraction from N¹ under atmospheric conditions primarily leads to 2,3,4,5-tetrahydropyridine and in minor amounts of 1-nitrosopiperidine and 1-nitropiperidine. The calculated rate coefficient for the piperidine + OH reaction agrees with the experimental value within 35%, and aligning the theoretical numbers to the experimental value results in k(T) = 2.46 × 10^-12 × exp(486 K/T) cm³ molecule^-1 s^-1 (200-400 K).